The present invention relates to a system for determining the forces exchanged by and/or the displacement of a movable member and particularly, although not exclusively, the linear and angular position of a body with respect to an axis of reference. The present invention finds particular utility as apparatus for the determination of the deformation properties of materials. Embodiments of the present invention may comprise contactless magnetic means for application of defined vector forces to a suspended body, and contactless means for the determination of vector displacements of that body. The system may include means for measuring time, to enable the accurate characterization of material deformation events, especially, but not exclusively, under low and high-pressure, conditions.
The study of material deformation events is common to many scientific and technological fields and precise description and measurement of such events has subsequently led to great improvement in materials production and usage.
Measurement of the deformation that results from a known applied force (stress) is fundamental to most such studies and usually expressed as the deformation (strain) induced by an applied stress. A contemporaneous measure of time provides an additional insight into most such deformation events.
Thus for example, time-dependent permanent (plastic) deformation or creep can occur when classical Hookean solid materials are subjected to stress below the proportionality limit over long periods.
The deformation of simple liquids, may be regarded as continuous permanent deformation (flow) and is expressed in terms of a shearing stress and a shear strain rate, while more complex visco-elastic materials may exhibit a degree of recoverable deformation that itself is time dependent.
Thixotropic behaviour is both rate and duration dependent.
Of the different instruments devised for deformation measurement, many deploy a common principle.
A known force is applied to a geometrically defined body in contact with a test material supported in a geometrically defined manner, so as to resist the applied force or stress. A measurement of the resultant material deformation as a linear or angular displacement is used to quantify the strain induced.
In liquids the strain is expressed as a rate; the velocity gradient between the adjacent shear planes.
Such simple devices as penetrometers which utilizes a gravity-driven plunger to apply force share the above common principle with complex instruments such as rotational rheometers and in consequence, share similar potential limitations to measurement and common sources of error.
In such instruments, the line of action and magnitude of the applied force must be defined (a force vector) and this usually calls for friction bearing or guides.
Friction arising from these bearings limits the accuracy of the applied force, while bearing-alignment error corrupts the geometric accuracy of the vector, as will errors in alignment of the surface which supports the test material.
A further common source of error or limitation lies in the accuracy and resolution of the measurement of displacement induced in the test material.
Others factors which may affect the integrity of test results, are those which might alter the physical or chemical nature of the test material.
Temperature and pressure, for example, can have a significant influence and should be at least monitored and preferably controlled during the test; indeed, in some cases, the temperature and pressure conditions of material storage prior to testing can significantly alter its subsequent deformation behaviour.
Sample changes due to the loss of volatile constituent are also a potential source of error.
Thus for an idealized test, it is required that:    1. the force application member (such as a plunger or rotary bobbin) has a geometrically accurate surface    2. the magnitude of the force applied to the application member should be known with the greatest possible accuracy    3. its subsequent movement should be controlled to close tolerances without any friction    4. the sample support surface should itself be geometrically accurate and its spatial relationship to the line of action of the actuator should also be constrained to close tolerances    5. the test conditions of temperature and pressure should be controlled and volatile loss prevented.    6. time is accurately measured during the test period.
The above represents a ‘minimal ideal’ but some complex forms of material behaviour may also require that sample deformation tendency be measured as more than one vector; such as investigation of normal stress effects that may be exhibited, for example, during extrusion processes as die swell.
Another common requirement is for repeated reversal of the direction of stress application, as in oscillatory testing, so as to reveal visco-elastic behaviour.
Other chemical and physical changes may occur as a result of testing and might be usefully indicated by contemporaneous measurement of hydrogen ion concentration (pH) or electrical conductivity for example.
In attempting to meet the above ideals, we have conceived a device based upon the contactless support of an appropriately shaped member by means of modulated magnetic forces, with the forces required to move the actuator being continuously measured or at least determined. Any movements (linear or rotary) of the member are also measured using non-contact position sensors and thus the member may be supported and moved, while free of any contact with other components of the device.
It may similarly be supported while enclosed within a chamber, without touching the chamber walls.
In this way, we avoid friction errors and enable the test member to have contact solely with the test material which may also be present within this chamber.
Pressure conditions may thus be controlled during testing and volatile loss avoided.
Rheometers may be regarded as among the more complex instruments used for measurement of material deformation and flow properties.
Rotary devices are commonly used as viscometers and rheometers; shear stress being defined in terms of a turning moment or torque and shear strain defined by an angular displacement.
A simultaneous measure of time allows for rate computation and determination of the rate of change of velocity of the shear planes within the sample with respect to the distance between the planes (shear strain rate). The coefficient of viscosity is the constant of proportionality that relates shear stress and shear strain rate.
While rotary viscometers and rheometers are both capable of the required determination of shear stress, shear strain and time, rheometers are distinguished by greater range, accuracy and flexibility in the mode of application and measurement of these parameters. Of particular significance is the ability to measure very small increments of shear stress and shear strain to allow investigation of limiting sample characteristics.
A further valuable rheometric function lies in the capability for controlled reversal of rotational direction (oscillation) which enables measurement of recoverable deformation (visco-elastic) behaviour in liquids and semi-solids. Rheometers also allow attenuation or adjustment of the defined applied variable (shear stress or shear strain) with respect to time and other dependent or independent measured variables such as, temperature, pressure, torque, angular displacement, pH, electro-potential and electro-conductivity, that might affect or be affected by, the sample under test.
The flow conditions that may be induced between adjacent defined surfaces, such as between cones, plates and cylinders, for example, have been well defined and are widely accepted as offering optimal compromises for accurate measurement.
However, such states of defined deformation and flow assume a constant radial or annular relationship between the adjacent surfaces whether or not there is relative angular displacement about a common axis.
Hence, high levels of dimensional accuracy are required both in the conformation of the components and in the maintenance of their juxtapositional relationship when static and during rotation.
As described earlier, any bearing system deployed to support the movable component or components therefore requires the best possible dynamic stiffness, while the precise application and measurement of angular displacement and torque can be corrupted by friction or drag arising from the support or drive mechanism of the instrument.
Thus, much instrument development has been aimed at improving bearing performance, especially the reduction or elimination of the deleterious effect of friction and drag.
To this end, non-contact devices have been designed in which bearing surfaces are separated by means of air currents (air bearings) or magnetic repulsive forces (magnetic bearings).
Since air bearings require that a constant uniform flow of air be supplied, they are not well suited to function within an environment that is subjected to very low or very high pressure or changes in pressure.
Unmodulated (passive) magnetic forces of attraction and repulsion demonstrate inherent instability when applied to bearing function and thus require the addition of other means to constrain that instability.
The use of electro-magnets in which the electric power is modulated in direct response to changes of position of the supported component such that the consequent change in magnetic force serves to adjust the position of that component so as to attain its required spatial location is known.
It follows therefore, that any active magnetic bearing must comprise both the electro-magnetic modulation means and means for detecting the position of the supported component, linked together to form a closed-loop negative feedback system. The functional efficiency and the accuracy attained in constraining the supported component within closely defined spatial limits, depends largely upon the response rate of the feedback loop.
Most rotary viscometers and rheometers provide support for the rotating components which are in contact with the sample under test, by the provision of a supporting spindle attached to the sample-contact component and extending beyond the sample zone to an attached bearing. Any component used to measure torque or rotation of the support spindle is also similarly located outside the sample medium.
There are however alternative examples of viscometer design which have effectively eliminated the need for a discrete supporting spindle, by submerging a geometrically defined element entirely within the sample material which is contained within a chamber, such that the sample medium entirely invests the bobbin. These instruments rely upon externally generated magnetic forces or internal sample forces such as buoyancy or pressure gradients, induced in the sample by rotation or oscillation, to provide support and location of the bobbin, free of contact with the sample chamber or any other component of the instrument.
One such prior art example describes a visco-densimeter, in which the mass of a magnetically sensitive cylindrical bobbin is supported in vertical axial alignment by an active magnetic suspension system in opposition to gravity.
The spatial position of the bobbin is detected optically so as to provide the required reference signal for the feedback loop.
This stable support system allows suspension the cylindrical bobbin out of contact with the walls of a larger static encapsulating concentric cylinder which may be filled with test material to form a sample chamber provided with closure means.
A well defined annulus is thus created between the bobbin and the concentric sample chamber, thus enabling constraint of the shearing conditions as the cylindrical bobbin is rotated.
A contactless means of rotation is provided by a variable, rotating electro-magnetic field generated within static coils, peripheral to the sample chamber. Rotation of the bobbin is enabled by means of the well established principles of an eddy current drive system. The turning moment thus induced in the bobbin is opposed by viscous drag imparted by the test sample and the resultant rotation attained by the bobbin is detected by a small sensing coil located within the sample chamber, which responds to a magnet fixed within the bobbin. The frequency and current applied to the drive system may then be compared with the bobbin rotation so as to derive a measure of viscous drag in the sample.
Apart from the closure mechanism for the sample chamber, the bobbin is the only component capable of spatial movement and that motion is limited to an angular displacement relative to the concentric sample chamber, about the common axis. By providing contactless measurable axial support and measurable rotational drive within a sealed sample chamber, this design offers much of the competence identified as desirable for an ‘idealised’ instrument; absence of unwanted friction drag, well defined conformation within the sample measurement zone, the option of controlling the pressure conditions within the sample chamber and the elimination of volatile sample loss and containment of sample hazard.
However, the prior art example cited above, has functional limitations in that the rotation detection system demonstrates limited angular resolution, the optical position detector is not suited for use with opaque sample media, the sample chamber wall thickness and thus the pressure limit is restricted by the need for magnetic transparency and the axial orientation of the magnetic bearing support constrains operational alignment of the instrument with respect to gravity.
It is current practice in commercial rheometers to use optical detection for determination of angular displacement of rotary components and this achieves very high levels of resolution with systems which are often described as optical code wheels. Such devices are unsuited for use when invested by an optically opaque medium.
Alternatives to optical sensing have been tried elsewhere in devices suitable for detection of linear or angular displacement, usually called proximity sensors, including the use of such sensors as the required position-detection components of magnetic bearings.
Contactless proximity sensors, capable of precise position sensing when the intervening space between the sensor and the detected body contains an optically opaque medium, include those responsive to changes in electrical capacitance, electrical inductance and magnetic flux density (Hall Effect devices), however, the prior art has not demonstrated the use of these for the precise measurement of angular displacement as required for the aforementioned ‘ideal instrument’.